Effects of zinc (Zn) and cadmium (Cd) interactions on root morphology and metal translocation in the hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of S. alfredii were investigated under hydroponic conditions. Specific root lengths (SRL), specific root surface areas (SRA) and specific root volumes (SRV) of the HE increased significantly when plant were treated with 500 mu M Zn or 100 mu M Cd + 500 mu M Zn, whereas these root parameters were significantly decreased for the NHE when plant were treated with 100 mu M Cd, 500 mu M Zn or 100 mu M Cd + 500 mu M Zn. SRL and SRA of the HE were mainly constituted by roots with diameter between 0.2-0.4 mm (diameter class 3 and 4) which significantly increased in treatment of 500 mu M Zn or 100 mu M Cd + 500 mu M Zn, whereas in the NHE metal treatments caused a significant decrease in SRL and SRA of the finest diameter class root (diameter between 0.1-0.3 mm). The HE of S. alfredii could maintain a fine, widely branched root system under contaminated conditions compared with the NHE. Relative root growth, net Cd uptake and translocation rate in the HE were significantly increased by adding 500 W Zn, as compared with the second growth period, where 100 mu M Cd was supplied alone. Cadmium and Zn concentrations in the shoots of the HE were 12-16 times and 22-27 times higher than those of the NHE under 100 mu W Cd + 500 mu M Zn combined treatment. These results indicate strong positive interactions of Zn and Cd occurred in the HE under 100 mu M Cd + 500 mu M Zn treatment and Cd uptake and translocation was enhanced by adding 500 mu M Zn. (C) 2009 Elsevier B.V. All rights reserved.